In certain separator applications, the separation fluid during the separation process is kept under special hygienic conditions and/or without any air entrainment and high shear forces, such as when the separated product is sensitive to such influence. Examples of that kind are separation of dairy products, beer and in biotechnology applications. For such applications, so called hermetic separators have been developed and in production for a number of years.
In a hermetic separator, the separator bowl or centrifuge rotor is completely filled with liquid during operation. This means that no air or free liquid surfaces is meant to be present in the bowl. As can be seen in FIG. 1 which discloses a previously known hermetic centrifugal separator, the fluid to be separated enters the centrifuge rotor 1 from the bottom through a hollow spindle pipe 2 forming a hermetic inlet 3. This provides a gentle acceleration of the fluid having shear-sensitive contents. The feed is then accelerated in a distributor 4 before entering a disc stack 5 comprising separator discs where the separation takes place. The liquid phase moves towards the center of the centrifuge rotor 1, where the liquid phase is pumped out under pressure by means of a built-in pump disc 6 to at least obtain a required outlet pressure. The separated heavier solids phase is collected at the periphery of the centrifuge rotor 1, from where the solids are discharged intermittently through solids ports 7.
However, the pressure drop inside the separator is not reduced. The main part of this pressure drop arises in the narrow section where the fluid passes the outlet sealing.
In order to create a flow of process fluid through a hermetic separator, an inlet pressure has to be provided to overcome the pressure drop in the separator. The inlet pressure required at a certain capacity is often higher than desired. This provides a problem, especially within areas with a requirement of soft treatment of the process fluid such as biotechnology. Thus, a contradiction between high capacity and quality of separation product appears.
In beer separation, the required inlet pressure can be high, caused by carbon dioxide leaving the fluid which in its turn may cause cavitation. It also leads to capacity problems, as the process fluid flow will be reduced.
A part of the pressure drop over the separator arises in the narrow section where the fluid passes the outlet sealing.